The evolving networks and new types of terminals, particularly smart phone type terminals, are gradually changing the characteristics of mobile communications traffic. There will be more and more applications requiring always-on type of connections. This trend gives rise to a number of challenges both in the radio access network load as well as in the terminal. The network may have issues with signaling load caused by a large number of connected UEs (user equipment) doing handovers or UEs changing the state between idle and connected.
Smart phones will generate traffic when unattended if certain applications are launched. Applications like social networking (e.g. Facebook), instant messaging (e.g., Nimbuzz, Skype) or the like will generate status updates, polling, “keep alive” or similar traffic even when the applications are not actively used. The traffic typically consists of small packets (or bursts of packets) sent intermittently. Also different operating systems (OS) may check various updates regularly.
Despite being “always-on,” the UE power consumption should be close to the idle mode power consumption when the services are not actively used. The UEs can be configured with DRX (discontinuous reception), or they can be placed into an idle state between the data transmissions allowing sufficient “sleep” times at the UE.
When optimizing the UE power consumption and/or network signaling load, the network should also allow appropriate service quality for data transfer when the user starts actively using a certain service/application. This gives rise to somewhat conflicting requirements for the UE configuration in order to be able to react fast enough to the coming data.
When in connected mode and with DRX configured, the UE is allocated uplink (UL) resources for possible radio resource requests (e.g., scheduling requests or SRs). The UL resource reservation is for the PUCCH (physical uplink control channel) and the reservation is kept as long as the UL synchronization is assumed to be valid, that is, the timing advance value (needed to align the UL transmissions from different terminals) is assumed to be within the allowed tolerance. The validity is controlled by a timer, namely a TAT, (time alignment timer), which is configured by the network. At the expiration of the timer, the UE releases the PUCCH resources and initiates the following data transfers with the RA (random access) procedure.
In LTE (long term evolution) or LTE-Advanced (LTE-A) networks the state transitions, DRX configuration and the usage of PUCCH resources are controlled by the network. To reach the optimum operation both from the UE and network perspective calls for more sophisticated features to be developed.
When in connected mode and the Time Alignment Timer (TAT) expires, the current procedure mandates that the UE release the PUCCH resources. The TAT needs to be kept up to date in order to keep UE uplink synchronization and the PUCCH resources allocated. Keeping the timer updated triggers continuous signaling (unless infinity time is configured by network). The network has to send a TA (timing advance) command MAC CE (medium access control element) before the TAT expires. Every time the TA command is sent, it activates the DRX inactivity timer which causes extra power consumption.
If the network lets TAT expire, PUCCH resources are released by the UE and UE has to start any uplink transmission with random access. After that the network has to signal the reconfiguration of the PUCCH resources to the UE with a RRC (radio resource control) Connection Reconfiguration procedure. A similar problem exists for the downlink transmission and the network needs to synchronize the UE UL (e.g., send a TA command typically using random access response), because otherwise the UE cannot transmit HARQ ACK (hybrid automatic repeat request acknowledgement) for the downlink transmission. FIG. 3 illustrates the contention based random access procedure after TAT has expired when the UE is in the connected mode and has C-RNTI (cell radio network temporary identifier) allocated. This procedure is used when UL data arrives in the UE. Contention resolution in this case is done with a PDCCH (physical downlink control channel) message to a UE's C-RNTI containing an UL grant for a new transmission (Msg 4, FIG. 3).
FIG. 4 illustrates the contention free random access procedure after TAT has expired. When the UE is in connected mode and has C-RNTI allocated. This procedure is used when DL data arrives in the UE after TAT expiry. It starts with a PDCCH order from eNB (evolved Node B) (Msg 0, FIG. 4). In this case no contention resolution is needed. Both figures indicate that RRC Connection Reconfiguration is needed after random access to reconfigure the PUCCH resources.